This invention relates to apparatus and systems for increasing the rate or product formation in spontaneous reduction-oxidation reactions, or for producing preferentially one particular product where alternate products are possible, in which the reactants are present in the form of two unmixed fluid phases in contact. Specifically, the invention relates directly to the art of catalysis and electrocatalysis in which a solid catalyst surface is provided at the interface between the reactants, which surface establishes the locus of the chemical reaction, the solid surface, at the interface between reactants being electronically conductive for optimum performance.